"We're rotating Roundup Ready corn with our soybean crop, which is typically 95% Roundup Ready," Johnson says. "With glyphosate being the primary herbicide used on soybeans, we simply are spraying it on a weed that it was not designed to kill."

With heavy, untreated infestations, the weed can cause up to a 40% yield reduction in soybeans or up to 30% in corn.

"Volunteer corn is more frequently a problem in fields where farmers use fall tillage, because it buries the corn seed and allows it to overwinter," Johnson says. "It is less of an issue in a strict no-till system because of rodents and weathering."

The weed also becomes more of a problem in fields where a lot of corn hit the ground during the previous harvest. If that's the case, Johnson says farmers need to scout before planting so they know what controls to employ and where.

Volunteer corn is fairly simple to control in soybeans because farmers can use post-grass herbicides. "We simply need to encourage growers to spray these a little bit earlier in the summer than we typically would," Johnson adds.

The weed is much more difficult to control in cornfields – especially those planted in continuous corn. The best methods are spring tillage and using non-glyphosate, preplant herbicides.

"Producers need to control volunteer corn before they plant their corn crops," Johnson says. "Spring tillage is more reliable, but if it's a no-till operation farmers need to spray."

Volunteer corn also can lead to significant corn rootworm problems.

"In the past, farmers rotated corn and soybeans. Soybean fields have zero rootworms because when the insects hatch, they have nothing to eat," says Purdue Extension Entomologist Christian Krupke. "With volunteer corn, those rootworms have something to feed on, so farmers need to kill this weed as early as possible."